Python Filtering technic described in the same post by King Tapir work pretty well and do not required that all of your material assigned on the object level. However, it might be difficult to figure out how to use it for the first time.

Made a .py file. In my case it called filter.py and contain following code.

Called just prior to the ray_end statement which locks off the settings for an instance object. The function can query object: settings and possibly alter them.

object:surface = ('')

The surface shader attached to the object.

vm_overridedetail / object:overridedetail = ('false')

When geometry has shaders defined on a per-primitive basis, this parameter will override these shaders and use only the object’s shader. This is useful when performing matte shading on objects.
Not supported for per-primitive material assignment (material SOP).

'op:/shop/AO'

Shop material that we going to override

Tell mantra to use this file as a Python Filter

Also, don’t forget to force writing of all shaders into .idf by setting Declare All SHOPs.